U.S. Provisional Application No. 61/900,154, filed Nov. 5, 2013, is incorporated herein by reference in its entirety.
Railroad Pneumatic Retarder Control Systems
Railroad classification yards break incoming trains into individual cars at the top of a hump (hill) where they are weighed and identified. As a car rolls down the hump by gravity, it is switched onto one of many parallel tracks to form new train strings according to their destination. Car speed is monitored as it rolls down the incline and at several points it passes through a pneumatic retarder, or rail brake, where an appropriate degree of braking is applied to regulate the car speed in proportion to its weight. The objective is for the car to have enough momentum to complete its journey but not so much that cargo is damaged when it contacts and connects to the car before it in the stationary string. It is desired for the car speed to be regulated as precisely as practical, as a non-limiting example, within just a few fractions of a mile per hour, as a reference example, 3.5 miles per hour.
The retarder (brake) replaces a section of track and is constructed so that air cylinders or air-bag actuators work through a pincer mechanism to force brake pads into contact with car wheels as they pass through. A typical retarder has multiple individual mechanisms, which are operated in unison so that braking is uniformly applied as a car passes through. The degree of retardation (braking) is determined by the amount of air pressure supplied by the control valves. An inlet air valve is turned on to pass air to the retarder system until a particular pressure is achieved then the valve is closed to prevent additional flow which would cause overpressure. When the rail car has been sufficiently slowed, one or more exhaust valves are opened to exhaust or dump retarder pressure to atmosphere. Exhausting the retarder takes longer than filling it so multiple exhaust valves are commonly used to reduce exhaust time. But the more precisely the exhaustion can be controlled, the more precise will be the braking effect achieved. In a typical retarder control system, operating air pressures are as high as about 100 psi over atmospheric pressure. Pressure differentials of this magnitude have been found to be problematic, as they can result in high air flow velocities that can exert high load conditions on the valves, particularly exhaust valves, resulting in excessive wear and breakage requiring service or replacement more frequently than desired.
Railroads employ sophisticated computer software in the control tower to weigh the cars, monitor car speed and apply the proper amount of air pressure (including supply and exhausting of the pressurized air) to retarders in order to regulate the speed of each car even though several cars may be rolling separately down the incline in close sequence. Each retarder is actuated by a nearby control box typically trackside at each retarder where air valves are operated either directly by a signal from the tower or that signal may be further processed by a programmable logic controller (PLC) before operating the air valves. It is desirable for any new retarder valve or system to be compatible with this entire infrastructure.
Originally, retarder control air valve assemblies typically consisted of commercially available solenoid-controlled air valves and large, e.g., 1½″ diameter, standard black iron plumbing components such as tees, elbows, unions and nipples and flanges. The result was a large, heavy, and difficult to mount assembly that was also difficult to maintain because large pipe wrenches were required for the 1½″ joints and because the solenoid air valves were built with many loose components such as seals, springs, and screws that could easily be lost during on-site repair under difficult weather conditions (retarder control boxes are located outdoors alongside train tracks so weather can range from extreme heat to extreme cold, plus rain, sleet, or snow). The valve assembly was generally mounted underneath a hinged cover to protect the electrical components from the elements. The solenoids of air supply valves were accessible when the cover was lifted but exhaust air valves were mounted underneath the stand bottom so high-volume exhaust air could be vented to atmosphere outside the cover and away from the electrical components. Low ground clearance under the stand bottom made maintenance of exhaust valves even more difficult because of poor accessibility.
An improved generation of retarder controls positioned the air valves inside a protective metal box with a hinged lid and changed to double pilot-operated inline-style air valves that were suspended via unions and flanges between two large bar manifolds with bottom ports for air, exhaust and system connections through the box bottom. This was an improvement in that the valves and electrical solenoids were kept clean and were sheltered from weather conditions. However, to repair a valve it had to be removed along with its associated plumbing to be worked on. Another drawback was that the solenoid pilot valves required a filtered air supply and 4-way pilot valves were required to operate the double-acting main air valves. So, for example, a four-valve circuit might have a minimum of 10 tube connections for air supply pilot lines plus a minimum of 16 tube connections for valve exhaust lines—many of these lines would have to be disconnected in order to remove a valve for repair and any of these lines could leak and affect system operation.
Other commercially available valves were tried but the valves either reacted too slowly (ball valves) or were insufficiently rugged for this application and failed prematurely. Failure in the case of poppet valves was observed to be primarily due to the poppet not being effectively guided to resist the high side forces being generated by high velocity airflow through the valve. A further problem was a front-seated valve design that would shift open if pilot pressure was lost, even if the tower control signal called for the valve to remain closed. Still another problem is spring breakage, which can occur from repeated use under varying environmental conditions, and force loads that can be generated when shifting under high differential pressure conditions, and also when a spring has a design feature and/or manufacturing defect that gives it a propensity to breakage.
Typical modern pneumatic retarder control systems employ bulky commercially available air-piloted valves that are connected via conventional screwed or flanged pipe fittings to form a manifold assembly having an air supply port, one or more retarder ports, and one or more exhaust ports. These can include valve seats machined directly into the valve body and are not fully renewed by installing a rebuild kit so a new valve must be purchased and installed if a seat, or other wearing surface, becomes damaged. In order to replace an individual valve, pilot line tubing must be disconnected and several large pipe fittings must be undone to remove the valve from the manifold assembly.
Of course, conventional valves may be repaired on-site using a rebuild kit involving many loose parts, lubricants, and seals that can be easily lost or contaminated with grit from the rail yard. However, this doesn't completely renew the valve because valve seats, bearings, and piston bores located in the valve body are not replaced and may be in poor condition.
In addition, to make the large pilot-operated air valves controllable electrically, separate solenoid-operated pilot valves are connected via tubing and fittings. Commonly, two pilot tubes are required for each large controlled valve plus air supply tubing for each solenoid pilot valve. So, for a retarder control with four large 2-way valves there could be twenty pilot line tubing connections (fittings) plus another eight large plumbing joints (unions) for a total of twenty-eight potential leak points.
Reference in the above regard: U.S. Pat. Nos. 7,325,567 B2; 6,123,096; 4,446,886; and 3,227,179. Reference also U.S. Patent publications: 2008/0237511 A1; 2008/0173840; 2005/0005811 A1; and 2004/0112208 A1.
To protect the retarder control valves from inclement weather and extreme temperatures, it is common practice to mount the whole valve manifold inside an enclosure to keep the solenoids dry and also control temperature via heaters or fans. This requires multiple enclosure holes for connecting air supply, retarder connections, and exhaust mufflers—all of which should be sealed in order to exclude moisture and dust from the enclosure.
A segmented manifold may simplify manufacturing and reduce costs somewhat but that approach would build-in undesirable potential leak points at every air joint and pilot line joint between segments.
Any maintenance performed in a classification (hump) yard is expensive because safety rules and common sense requires a “watcher” to warn the worker of an approaching rail car. Because retarder controls are located at trackside there is no shelter from inclement weather so faster repair time is very desirable. Prior air valve designs have several common characteristics: they employ springs (internal or external) to return the valve to a “home” or rest position; the moving components such as spools and poppets are either not guided or an external bearing is required for alignment.
Poppet Valves
Large poppet valves are shifted by pilot air pressure acting on a piston which provides a high shifting force to the piston-spool-poppet assembly. Air flow capacity of a poppet valve is determined primarily by the diameter of the poppet and bore of the seat, and secondarily, by the distance the poppet moves away from the seat. As the poppet diameter is increased, to gain greater flow capacity, it becomes more critical that the poppet and seat be aligned so the closing force is spread uniformly over the entire circumference of the poppet. If any cocking or misalignment occurs, then the closing force is concentrated on a small initial contact area which can cause sealing problems and also shorten the life of the poppet seal which is typically rubber. Side loads are inherent in an air control valve because the air path typically involves a sharp or high angle turn, e.g., 90-degree turn, as it flows past the poppet—and the larger the poppet diameter, the larger are the side loads produced by high-speed air flow. This, combined with the high pressures used in retarder control systems cause the poppet to be pushed out of alignment with the valve seat, and also increases wear on the side load bearing components supporting the poppet, namely, the spool and associated structure. The resulting wear can also result in greater friction during movement of the poppet so that operation is less smooth. The known poppet valves are also known to suffer from the spring breakage issues noted above.
Typical known pilot-operated air valves in retarder control systems are operated by a pilot air signal from a small 3-way solenoid valve or other suitable valve. When the positive pilot air pressure becomes high enough, the large pilot-operated valve will shift. When the pilot air signal is exhausted to atmospheric pressure, the pilot-operated valve will be biased back to its home (normal) position by spring force. It is desirable to have a strong bias to home position to assure that no flow will pass through the valve that could cause a false retarder operation or cause the retarder brake shoes to prematurely move to closed position before a rail car is present so as to possibly cause a derailment or other problem. However, strong biasing springs can impart side loads to moving valve members and cause premature wear, and they can also pose a risk of injury to a mechanic during assembly or disassembly of the valve mechanism. In light of these and the other spring related problems discussed above, it would be desirable to eliminate springs from the valve assembly.
Typical modern retarders generally comprise a scissors or comparable mechanism incorporated between the rails of a section of track, the mechanism incorporating air cylinders, or more recently, multiple air bellows type actuators, inflatable by the pressurized air supplied by the retarder control valve or valves, to press brake shoes of the mechanism against the insides of flanges of the wheels of a rail car as they roll over the track section, to slow the speed of the rail car to a desired extent. The bellows type air actuators have a large air volume. The volume is such that the 1½″ diameter hoses and piping connecting the bellows to the air valve manifold act as a restriction within the air flow system, both when inflating the bellows and exhausting the air therefrom. It has been found that having a restriction between the air valve manifold and the actuators results in delays between air valve operation and actuator response, typically manifold pressure being initially higher than pressure in the actuators when an air valve opens to pressurize the system. These pressures will equalize with time, as the higher pressure reaches the actuators. Similarly, manifold pressure will be initially lower than actuator pressure when an exhaust valve or valves open, but will equalize as the air is exhausted from the actuators. Thus it can be observed that there will be a phase difference between the pressures in the valves and the actuators until sufficient time passes for the pressures to equalize.
Pressure at the actuator(s), e.g., bellows is what determines the degree of retardation, and pressure feedback is required to enable precise control of air valves for controlling the air flow to and from the actuators. It would thus be desirable to locate a pressure feedback transducer or other sensor directly on or proximate to the actuator(s). However, this would place the transducer and attendant wiring vulnerable to weather and mechanical damage in a busy rail yard. To protect it and require less vulnerable wiring, it would be desirable to locate the transducer in or adjacent to the control enclosure and directly connect to the air valve manifold. But, direct connection to the air valves would potentially subject the transducer to any pressure spikes and shock waves generated when the valves open and close, and to the higher and lower pressures in the manifold compared to those in the actuator(s), due to the lag in pressure changes between the manifold and the actuator(s) from the piping restrictions, so that there is a danger of erroneous pressure signals being generated and responded to.
Thus, what is sought is an improvement in a valve arrangement for a retarder control system that overcomes one or more of the problems and shortcomings set forth above.